BEIRUT — A large explosion rocked a stronghold of the Shiite militant Hezbollah group south of the Lebanese capital today, setting several cars on fire, sending a thick plume of black smoke billowing into the sky and wounding at least 18 people, security officials said.

The powerful blast in a bustling commercial and residential neighborhood came as many Lebanese Shiites began observing the holy month of Ramadan, and is the worst explosion to hit the area in years — likely direct fallout of the civil war raging in neighboring Syria.

Groups of outraged Hezbollah supporters marched in the area after the blast, carrying pictures of Hezbollah chief Sheik Hassan Nasrallah and shouting slogans in support of their leader. Hezbollah operatives fired in the air to disperse people who attacked the interior minister with stones after he inspected the scene of the blast, trapping him for 45 minutes in a building before he was escorted through a backdoor.

With skirmishes between Shiites and Sunnis on the rise around the country, religiously mixed and dangerously fragile Lebanon is increasingly buffeted by powerful forces that are dividing the Arab world along sectarian lines. Some Syrian rebel groups, which are predominantly Sunni, have threatened to strike in Lebanon after Hezbollah joined Syrian President Bashar Assad’s troops in their battle against opposition fighters.

“This is a message, but we will not bow,” said Ziad Waked, a municipal official speaking to Hezbollah’s Al-Manar television.

Today’s explosion struck the area of Beir el-Abed, and was most likely caused by a car bomb, officials said on condition of anonymity in line with regulations. They said the blast was in the parking near the Islamic Coop, a supermarket usually packed with shoppers, and a petrol station.

“I was riding my motorcycle on my way to a sweets shop and then there was this massive explosion that knocked me off and I fell on the ground,” said a 52-year-old employee of a private company.

“At first I thought it was an earthquake,” said the man who declined to be named out of security concerns. He said he was fasting on the first day of Ramadan and was on his way to shop for the evening meal that would break his daylong fast.

Al-Manar and security officials said 18 people were wounded. Red Cross head of operations George Kattaneh put the number at 37, saying they were all light injuries, many of them from breaking glass.

The area is near what is known as Hezbollah’s “security square” where many of the party’s officials live and have offices. today’s explosion is one of the biggest in the area since the end of the country’s 15-year civil war, which ended in 1990.

Television footage from the scene of the explosion revived memories of that conflict, when car bombs set by sectarian groups were common. There have been numerous car bombs targeting politicians and journalists since then, but random car bombs have been rare.

Hezbollah operatives in civilian clothes, some of them carrying Kalashnikov rifles, cordoned off the site of the explosion with yellow ribbons. They and Lebanese security officials barred journalists from approaching the site itself.

Ambulances and fire engines, their sirens wailing, raced to the area and witnesses said casualties were rushed to the nearby Bahman and Rasoul al-Atham hospitals. Immediately after the blast, people could be seen running in the street away from the site of the explosion which set several cars on fire.

The power of the explosion shattered windows and damaged several buildings in the busy residential and commercial area.

In May, two rockets slammed into a Hezbollah stronghold in south Beirut, wounding four people. The rockets struck hours after Hezbollah leader Sheikh Hassan Nasrallah vowed in a speech to help propel Assad to victory in Syria’s civil war.

In June, a rocket slammed into the same area, causing no casualties.

Hezbollah has openly joined the fight in Syria, and the group’s fighters were instrumental in a recent regime victory when government forces regained control of the strategic town of Qusair near the Lebanese border.

Lebanon’s Sunni Muslims mostly back the overwhelmingly Sunni rebels in Syria, while many Shiites support Assad, who is a member of Syria’s minority Alawite sect, an offshoot of Shiite Islam.

“It is not a surprise for Dahyeh, the stronghold of resistance, to be targeted by such lowly, treacherous attacks that bear the fingerprints of the Israeli enemy and its tools,” said Hezbollah lawmaker Ali Ammar, referring to the name by which the suburb south of Beirut is known.

Hezbollah, much like the Syrian regime, accuses Syrian rebels of being agents of the U.S. and Israel.

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Associated Press writer Zeina Karam contributed to this report.

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